Forget spring and fall. The planet KELT-9b only has winter and summer.
And that is just one of the remarkable conclusions that scientists can draw now that they have examined perhaps one of the strangest planetary systems ever. We are talking about the star KELT-9 and its planet KELT-9b, both of which enter the books as – and to put it mildly – extremely peculiar.
About the planet
KELT-9b was discovered in 2017 and orbits KELT-9 670 light-years from Earth. The planet is about 1.8 times larger than Jupiter and is so close to its parent star that it only takes 36 hours to complete a circle around the star.
Partly because the distance between the star and planet is so small, the temperatures on KELT-9b are very high. On the day side – the side of the planet that is continuously aimed at the parent star by synchronous rotation – temperatures rise to 4300 degrees Celsius. And with that, the planet is even warmer than some stars! Earlier this year, research using the Spitzer space telescope found those temperatures even molecules on the planet go too far; they are literally torn apart by the heat!
An artistic impression of KELT-9 and KELT-9b. The system uses synchronous rotation. The time it takes for the planet to complete a circle around its own axis corresponds to the time it takes to complete a circle around KELT-9. This means that the same side of the planet always faces the star. This side is therefore the warmest and is called the day side. Image: NASA / JPL-Caltech.
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Temperature differences on the star
As if all that was not special enough, researchers have now discovered that the system has even more remarkable properties. It turns out that the star – which is about twice as big and 56 times warmer than our sun – turns a whopping 38 times faster than our parent star. This means that the star can complete a circle around its own axis in 16 hours. Because the star rotates so fast, it is exaggeratedly somewhat oval-shaped: the poles are, as it were, pushed in and the middle part of the star widens. This in turn causes the poles to heat up (and brighten), while the area around the star’s equator cools and dims. It results in a huge temperature difference between the poles and the area around the equator that can reach 1500 degrees Celsius!
Short-lived seasons
And that also affects the planet. The orbit of KELT-9b carries it over the area near the equator and over the poles. The result is that it takes full account of the temperature differences on the surface of the star. Every time the planet moves over the poles, the temperature rises and goes through a ‘summer’. While a movement over the equator results in relatively lower temperatures and thus a ‘winter’ season. Since the planet takes just under 36 hours to complete a circle around the star, it spans four seasons in about a day and a half: two summers and two winters. And each season lasts about nine hours.
Atmosphere
“It’s intriguing to think about how temperature differences on the star affect the planet,” said researcher Knicole Colón, of NASA’s Goddard Space Flight Center. “The variations in the amount of energy that the planet receives from the star must lead to an extremely dynamic atmosphere.” This atmosphere is also subject to major changes anyway. For example, researchers previously showed that the high temperatures on KELT-9b cause the atmosphere to ‘boil’ and escape into space. In 2018, researchers still estimate that the atmosphere of KELT-9b in this way lose more than 100,000 tons of hydrogen per second.
Difficult to study
“KELT-9b is really crazy,” said researcher John Ahlers, of NASA’s Goddard Space Flight Center. “It is a massive planet in a narrow, almost polar orbit around a rapidly rotating star, and all of these properties make it more difficult to understand the star and its effects on the planet.”
KELT-9b was discovered using the transit method. Stars are looked at for a long time, in the hope that their brightness will decrease regularly. Indeed, such a decrease in brightness may indicate the presence of a planet that occasionally stands between the star and us, thereby blocking part of the starlight. Using this method, you can normally also learn more about the star and planet and its atmosphere. But as Ahlers already pointed out, things are a bit trickier with KELT-9. For example, the brightness of the star varies considerably, because the poles are much warmer than the area near the equator. And then you also have to deal with a planet with a special orbit, which transports the celestial body over both the equator and the poles. Thanks to TESS, researchers have now managed to learn more about KELT-9 and KELT-9b. He observed KELT-9 very intensively between July 18 and September 11, 2019. The planet hunter saw KELT-9b pass 27 times in front of KELT-9 and took measurements every two minutes. And thus reveals that the system is even stranger than we thought.
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